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E2.3 · Predict and test images formed by mirrors

Learn to predict and test images formed by mirrors through clear examples and targeted practice.

Ontario Grade 10 Science

Physics: Light and Geometric Optics

Use observations and simple ray models to describe what a mirror shows.

A mirror can make your face look upright, smaller, or even upside down. The result depends on the mirror’s shape and on where the object is placed. In this lesson, an object is the person or thing being viewed. An image is the appearance formed by light after it reflects from a mirror. You will first use familiar observations, then make predictions with a simple model, and finally compare predictions with evidence.

What you will learn

Describe an image using its orientation, relative size, and location.
Predict how a plane, concave, or convex mirror forms an image.
Use a simple ray model to explain and test a mirror prediction.
Record observations clearly and follow safe classroom procedures.

1. From seeing to describing

In Grade 9 science, you learned that light travels in straight lines and can reflect from a surface. Reflection is the change in direction of light when it strikes a surface and bounces away. A mirror lets us see an image because light from an object reflects into our eyes.

Try a safe classroom observation with a small plane mirror, which has a flat reflecting surface. Hold a printed letter in front of it. The image appears upright, about the same size as the letter, and behind the mirror. The image is not a second letter sitting behind the glass. It appears to come from there because of the way reflected light reaches your eyes.

Describe an image with three useful features. Orientation tells whether it is upright or upside down. Relative size tells whether it looks larger, smaller, or about the same size as the object. Location tells where it appears in relation to the mirror and object. For a plane mirror, the image appears as far behind the mirror as the object is in front. This is an apparent location, not a place where the image can be caught on a screen.

A ray is a line used to represent the path of light. A ray diagram is a drawing that uses rays to show how light travels and reflects. At the point where a ray strikes a mirror, imagine a line at a right angle to the surface. This is the normal. The angle between the incoming ray and the normal equals the angle between the reflected ray and the normal. Angles are measured from the normal, not from the mirror surface.

A mirror image is formed by reflected light reaching the eye.
Describe images by orientation, relative size, and apparent location.
For reflection, the incoming and reflected angles are equal when measured from the normal.

2. Mirror shape changes the image

A curved mirror has a reflecting surface that bends rather than staying flat. A concave mirror curves inward, like the inside of a spoon. A convex mirror curves outward, like the back of a spoon. These shapes change the directions of reflected rays, so they can change the image.

A concave mirror can make a nearby object look upright and enlarged. At other object distances, it can form an upside-down image. A useful way to understand this change is to consider parallel rays: rays that travel side by side toward a concave mirror reflect toward one another. The place where they meet is called the focal point. It is a feature of the mirror, not a physical object. The distance from the mirror to this point is called the focal length.

When an object is farther from a concave mirror than its focal point, reflected rays can actually meet in front of the mirror. The resulting image is upside down and can be projected onto a screen. This is a real image. When an object is between the mirror and its focal point, the reflected rays spread apart. Your eyes trace them backward and the image appears behind the mirror. This is a virtual image. A virtual image cannot be projected onto a screen.

A convex mirror spreads reflected rays apart. The image appears upright and smaller, behind the mirror. It is virtual. Convex mirrors show a wider area than a plane mirror, which is why they are useful for viewing a broad space. The image looks smaller, so objects may seem farther away than they are.

These are general predictions, not a substitute for testing a particular setup. The object’s distance matters for a concave mirror. A simple classroom ray sketch can help: draw two rays from the top of the object to the mirror, reflect them using equal angles, and check whether the reflected rays meet or spread apart. Do not treat a sketch as a measurement. Compare its prediction with what you actually observe.

Concave mirrors can form different images depending on object distance.
A real image can be projected onto a screen; a virtual image cannot.
Convex mirrors form upright, smaller virtual images.

3. Make a prediction, then test it

A good test begins with a prediction that can be checked. Choose one mirror and one object. State what you expect about the image’s orientation, relative size, and apparent location. Keep the object and mirror steady while you observe. If you change the object distance, record that change because it can affect the result, especially with a concave mirror.

Use classroom equipment as directed by your teacher. A small mirror and a printed object are suitable for observing images. A screen can be used to test whether a suspected real image can be projected. Never look at the Sun or use a mirror to direct sunlight into someone’s eyes. Handle glass mirrors carefully, and tell your teacher about a chipped or broken mirror. Do not use a bright lamp or other intense light source unless your teacher has approved the setup.

Record what you saw rather than changing your notes to match your prediction. If the result differs, check the mirror type, object position, and viewing direction. Repeat the observation when it is safe and practical. A test gives evidence about that setup; a single observation does not show that every mirror of the same type will look identical under all conditions.

The examples below use hypothetical classroom observations. They are predictions and sample records, not claimed experimental results. In each case, notice how the mirror type and object position guide the prediction, and how the test checks it.

Write a prediction before observing.
Change one setup feature at a time when comparing observations.
Record actual observations honestly and follow teacher safety directions.

Worked example

A plane-mirror letter

A letter is held 18 cm in front of a plane mirror. Predict the image’s orientation, size, and apparent location. Then describe a safe way to test the prediction.

Identify the mirror
A plane mirror has a flat surface. Its image appears upright and about the same size as the object.
Predict the location
For a plane mirror, the apparent image distance behind the mirror matches the object distance in front. So an object 18 cm in front has an image that appears 18 cm behind it.
$18\text{ cm}$
Test the prediction
Hold the letter steady in front of the mirror and compare its image with the letter. Record its orientation and apparent size. Do not claim that the image is physically behind the mirror; that is where it appears to be.

Answer: The image is upright, about the same size as the letter, and appears 18 cm behind the mirror.

Check: The location follows the plane-mirror rule: equal apparent distances on opposite sides of the mirror.

Worked example

A nearby object in a concave mirror

A small object is placed close to a concave mirror, between the mirror and its focal point. Predict what a viewer should see. How could a screen test help?

Use the object position
The object is between a concave mirror and its focal point. In this position, reflected rays spread apart rather than meeting in front of the mirror.
Predict the image
Because the reflected rays spread apart, the image appears behind the mirror. It is virtual and upright, and it appears enlarged.
Check with a screen
Place a screen where directed by the teacher and look for a focused image. This predicted virtual image cannot be projected onto the screen. Seeing no image on the screen is consistent with the prediction, but careful setup and observation still matter.

Answer: The image appears upright and enlarged behind the mirror. It is virtual, so it cannot be projected onto a screen.

Check: The object is closer to the mirror than the focal point, so the reflected rays do not meet in front of the mirror.

Worked example

Comparing a convex mirror with a plane mirror

A student views the same small object first in a plane mirror and then in a convex mirror. Predict one difference in the images and one difference in the view of the surroundings.

Recall the plane-mirror image
A plane mirror makes an upright image about the same size as the object.
Apply the convex-mirror pattern
A convex mirror makes an upright, smaller, virtual image. Its outward curve spreads reflected rays apart.
Compare the view
The convex mirror can show a wider area than the plane mirror. The object’s image looks smaller, so the student should not assume that the object is as far away as it appears.

Answer: The convex-mirror image is smaller but upright, while the plane-mirror image is about the object’s size. The convex mirror also shows a wider area.

Check: Both predictions follow from the image patterns for plane and convex mirrors.

Common mistakes and how to avoid them

Saying every mirror image is upright.

Correction: A concave mirror can form an upside-down image when the object is farther from the mirror than the focal point.

Calling an image real just because it looks clear.

Correction: A real image is formed where reflected rays meet and can be projected onto a screen. A virtual image only appears to come from a location.

Measuring the reflection angle from the mirror surface.

Correction: Compare the incoming and reflected angles from the normal, the line at a right angle to the surface.

Treating the apparent position behind a plane mirror as a physical location.

Correction: The image appears behind the mirror, but it is not an object located there.

Lesson summary

Light reflects from mirrors and enters the viewer’s eyes to form an image.
A plane mirror forms an upright image about the same size as the object, appearing as far behind the mirror as the object is in front.
A concave mirror’s image depends on object position: it can be an enlarged upright virtual image nearby or an upside-down real image farther away.
A convex mirror forms an upright, smaller virtual image and shows a wide area.
Make a prediction, test it safely, and report what you observe.

Check your understanding

Question 1

A student places an object close to a concave mirror, between the mirror and its focal point. Which prediction best fits the model?

An upright, enlarged virtual image appears behind the mirror.
An upside-down real image appears behind the mirror and can be projected there.
A smaller upright image appears in front of the mirror and can be projected onto a screen.
The mirror forms no image because the object is inside the focal point.

Show answer and explanation

An upright, enlarged virtual image appears behind the mirror.

In this position, reflected rays spread apart. The image appears behind the mirror, upright and enlarged, and cannot be projected onto a screen.

Question 2

An object is 11 cm in front of a plane mirror. Where does its image appear?

About 11 cm behind the mirror.
About 11 cm in front of the mirror.
At the mirror surface.
At a distance that cannot be predicted for a plane mirror.

Show answer and explanation

About 11 cm behind the mirror.

A plane-mirror image appears as far behind the mirror as the object is in front.

Question 3

Which statement best describes a convex mirror?

It forms an upright, smaller virtual image and shows a wide area.
It always forms a same-size image that can be projected onto a screen.
It forms an enlarged upside-down image for every object position.
It shows a narrower area because it brings reflected rays together.

Show answer and explanation

It forms an upright, smaller virtual image and shows a wide area.

A convex mirror spreads reflected rays apart, producing an upright, smaller virtual image and a broad view.

Key terms

Image: The appearance formed when light from an object reflects from a mirror and reaches the eye.
Ray: A line used to represent the path of light.
Normal: A line at a right angle to a surface at the point where a ray strikes.
Concave mirror: A curved mirror whose reflecting surface bends inward.
Convex mirror: A curved mirror whose reflecting surface bends outward.
Focal point: The point toward which parallel rays reflect from a concave mirror.
Real image: An image formed where reflected rays meet; it can be projected onto a screen.
Virtual image: An image that appears to come from a location where reflected rays do not actually meet; it cannot be projected onto a screen.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation E2.3. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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